External donors are used together with Ziegler catalysts in the polymerization of propylene for influencing the properties of the final product. For example, external donors may be used to influence or control the molecular weight (Mn, Mw, Mz, MWD), tacticity, the amount of xylene soluble material in the polymer, and other parameters of the product.
One example of an external donor used in the polymerization of propylene is silanes. For example, US20110152424 discloses use of a methyl(cyclohexyl)dimethoxysilane (C-Donor) electron donor system. Various other dialkoxydialkylsilanes such as diisopropyldimethoxysilane (P-Donor, EP0850889 and U.S. Pat. No. 4,829,038) isobutyl(isopropyl)dimethoxysilane (BUPS, EP0850889 and U.S. Pat. No. 4,829,038) and dicyclopentyldimethoxysilane (D-Donor, U.S. Pat. No. 4,990,479 and U.S. Pat. No. 5,438,110) have also been used as external donors (illustration below).

The C-Donor is known in the art to produce a fair amount of xylene solubles (XS). It allows to adjust the XS from about 1 to 5 wt % due to a moderate silane response. As used herein, the term “silane response” refers to the response of the XS to variable amounts of silane. It is represented by the m(silane)-XS plot (FIG. 3). It is understood by one skilled in the art, that a steep silane response does not allow a reliable and constant XS output on the industrial scale. Therefore a more smoother response as seen in the C-Donor is beneficial.
In the art hydrogen is used to control the melt flow index (MFR) of the polymer. In addition to the MFR, also the productivity of the catalyst system is influenced. The productivity of a given system must therefore be regarded in relation to the MFR produced. With the C-donor, very low productivites are observed at low MFR.
Bulky silanes, such as BUPS, P-donor and D-donor, offer good activities at low MFR. However, this group of silanes produces very low XS (<1). Combined with a steep silane response (FIG. 3), the XS cannot be adjusted reliably using these donors.
Thus, there continues to be a need for catalyst systems and donors which produce polypropylene with low MFR and high XS at good productivities. Such polymers exhibit high stiffness combined with good processability in applications such as pipe, sheet, raffia, film, blow molding, injection stretch blow molding or thermoforming
Furthermore, the synthesis of bulky silanes usually proves to be difficult and expensive. For example, BUPS is synthesised from expensive organometallic reagents. The popular D-Donor can be afforded through the more economical hydrosilylation route, but additional activators need to be utilised (e.g. EP0602922).
Thus, there continues to be a need for facile and economic routes to bulky silanes.
Bulky polycyclic alkyl substituents, such as bicyclo[2.2.1]heptan-2-yl derivatives, have been used in dialkoxydialkylsilanes earlier. In JP2521676 and EP0299712 bicyclo[2.2.1]heptan-2-yldimethoxy(methyl)silane (BDMMS) and bicyclo[2.2.1]heptan-2-yltrimethoxysilane (BTMS) have been disclosed.